A Couple Questions.. (about power supply and video card)

I would just like to get some knowledge from people who know more about this subject before I spend 450 bucks. At first I just needed and graphics card but then realized I need a lot better power supply. Now is there requirements that I should be aware of. I plan on buying this http://www.newegg.com/Product/Product.aspx?Item=N82E16817703009
Now this says its 7.4 x 5.9 x 3.4
That 7.4 is length, right? Because if it means that my computer needs to be 8 inches wide then we got a problem. I'm hoping theres no requirements for me to run this.
I also plan on buying Radeon hd 4870 I'm pretty sure that will be fine but if I wanted to do crossfire in the future would I be able to do this with the power supply? If theres anything here than you can answer or something I didnt know about this then please do post.

Thx a lot for the comment and I looked into it and my tower is indeed 7 inches wide. If that means i cant fit it then that sucks for me. I currently got a 250 watt and heard that means I got a small case. So is it easy, and i mean very easy to get and switch everything over to another case? On newegg I found a couple a 30 bucks. Is there any reason why I should spend more? I sort of want it to last me but I would be fine just using a cheap one for a while until i spend a lot on a more expensive one. And again, is it easy to move my motherboard and everything over to the new case? As much information as you want to give me is greatly appreciated.

it is pretty easy to move everything to a new case as long as you just hook everything back up the way it was. if you have never done anything like this before my suggestion is take a few pictures of the old case with all the components still hooked up. make sure you get a pic of how the on/off switch, hdd light and all those little tiny wires that are plugged directly into the mobo are all hooked up. once you have some pictures of what it is supposed to look like then just start carefully taking one piece out at a time and lay them some where, where they won't get damaged. put the mother board in first, then CD drive then hard drive then expansion cards then new PSU when it gets there.